1. Field of the Invention
The present invention relates to a liquid jetting head unit and an image forming apparatus.
2. Description of the Related Art
There are image forming apparatuses such as printers, fax machines, copiers, plotters, and multifunction peripherals including functions of these devices. An inkjet recording apparatus is known as an example of an image forming apparatus of a liquid jet recording method using recording heads configured with liquid jetting heads (liquid droplet jetting heads) for jetting ink droplets. Such an image forming apparatus of a liquid jet recording method forms images (record and print may be used synonymously as form) by jetting ink droplets from recording heads onto a conveyed sheet (the sheet is not limited to a paper sheet; the sheet may be any sheet onto which ink droplets or other types of liquid can adhere such as an OHP transparency film; the sheet may also be referred to as a recording medium, a recording sheet, etc.). There are several types of image forming apparatuses of a liquid jet recording method. One example is a serial type image forming apparatus that forms images by jetting liquid droplets while moving the recording heads in a main scanning direction. Another example is a line type image forming apparatus that uses line type heads to form images by jetting liquid droplets while the recording heads do not move.
In the present application, an image forming apparatus of a liquid jet recording method means an apparatus that forms images by jetting liquid onto a medium such as paper, thread, fiber, cloth, leather, metal, plastic, glass, wood, ceramics, etc. Forming images on a medium means forming images having meaning (such as characters and figures) and forming images without any meaning (such as patterns, e.g., merely jetting liquid droplets onto a medium). Ink is not limited to what is generally referred to as ink; ink refers to any kind of liquid that can be used for forming images, such as recording liquid and fixing process liquid; examples of ink are DNA samples, resist, and pattern material. Furthermore, an image is not limited to a planar image, an image may be formed on a three-dimensional object, or a three-dimensional object may be formed.
In an image forming apparatus of a liquid jet recording method, when air bubbles are mixed in the liquid jetting head, the droplet jetting direction may change or droplet jetting failures may occur. Therefore, it is necessary to efficiently discharge the air bubbles that have been mixed in the ink supply path.
Conventionally, there is a technology for discharging air bubbles in a head tank (a tank directly provided in the head; may also be synonymously referred to as a sub tank or a buffer tank). For example, a recording head is equipped with a first ink chamber having an indraft port for intaking ink from an ink supply source, a second ink chamber to which the ink is supplied from the first ink chamber, an outlet port for jetting the ink of the second ink chamber to perform recording, a first exhaust port for exhausting the liquid from the first ink chamber, a second exhaust port for exhausting the liquid from the second ink chamber, and a gas-liquid separation means for regulating the exhausting of the liquid, which is installed at least in one of the spaces between the first ink chamber and the first exhaust port, and between the second ink chamber and the second exhaust port. The fluid resistance between the indraft port and the first exhaust port is smaller than the fluid resistance between the indraft port and the second exhaust port (see patent document 1).
In another example of a recording head, the top face of a flat first space extending along the filter longitudinal direction which is a space in the upstream of the filter of a filter chamber, is formed as a slope which inclines downward from one side of the direction of a nozzle row toward the other end side, namely toward the side corresponding to the exhaust port from a feed port side. The bubble exhaust port that is facing the first space and that is used as a bubble exhaust path for exhausting bubbles, is provided at a position corresponding to the upper part on the side opposite to the exhaust port through the filter (see patent document 2).
In yet another example, an image forming apparatus includes a first ink flow-in port, a second ink flow-in port and a third ink flow-in/out port communicating with a common liquid chamber. An opening/closing valve which blocks a passage connecting a sub tank and the first ink flow-in port is provided. In a printing mode, the ink is fed to the recording head from the sub tank through the first ink flow-in port or through the first ink flow-in port as well as an ink flow-in/out port. In an ink circulation mode, the ink is fed to the recording head from the sub tank through the second ink flow-in port by closing the opening/closing valve, and the ink which flows out from the recording head is collected in the sub tank through the ink flow-in/out port (see patent document 3).    Patent Document 1: Japanese Laid-Open Patent Application No. 2009-126044    Patent Document 2: Japanese Laid-Open Patent Application No. 2008-030333    Patent Document 3: Japanese Laid-Open Patent Application No. 2006-168023
For example, a line-type image forming apparatus includes a recording head unit having a width extending across the entire width of a recording medium. In the recording head unit, plural liquid jetting head units are aligned. Each liquid jetting head unit is formed by integrally combining a head and a tank for supplying liquid to the head (head tank). Ink is supplied from the main tank to the head tanks through a sub tank and distributors for distributing ink to the heads. The head tanks include filter members for filtering impurities mixed in the ink.
However, the nozzle of the head is extremely fine (for example, φ24 μm). Therefore, if the ink includes a large amount of dissolved oxygen, oxygen gradually accumulates in the head, which leads to ink jetting failures. If air bubbles are mixed in the ink in the ink supply path including the head tank, the ink with air bubbles is distributed to the head, and the amount of dissolved oxygen in the ink increases. Consequently, the head may not be able to jet a predetermined amount of liquid droplets, or the head may not be able jet any liquid droplets at all. Thus, it is necessary to remove the air bubbles from the ink.
As described above, if air bubbles are generated in the head tank and the air bubbles are mixed into the head, liquid droplets may not be properly jetted from the head.